isn't google fabulous?
"The original name was Greek "Stephanos", borrowed into other European languages as "Stephan" or "Stephen"
"ph" was an ancient Greek way of indicating the "f" sound
(The Greeks came up with this method of combining two letters to indicate one sound [called a "digraph"] for situations where the Phoenecian alphabet they had borrowed their letters from didn't have a letter to suit their needs. This is the source of a whole group of these ch, sh, th, ph. The Greeks LATER invented separate letters instead chi (looks like X), theta and phi. But the LATIN alphabet was based on an OLD Greek alphabet, and they ended up continuing the digraph practice. In the case of the f-sound, they actually had their own letter (and ours) F, but whenever they borrowed a word from Greek they would use "ph" to indicate the sound. English continued this practice; many Romance languages simply shifted to "f" -- hence English "telephone" [from Greek roots] is "telefon" in Spanish.)
Now in English, over time, the NAME "Stephen" changed pronunciation a bit so that the "ph" sound was "voiced" (I think because of the the change in the VOWEL sounds to two long e sounds)-- changing from /f/ to /v/. But it is common with NAMES to retain traditional spellings even when pronunciation changes. Thus "Stephen" continued to be the normal spelling, and many still use it today.
Alongside this though (as with many other names) OTHER folks decided to change the spelling to better reflect the pronunciation. Thus the more 'modern' spelling "Steven"
Note that there are LOTS of reasons for 'new' spellings of names. Some are just to be different. But in THIS case there was a very logical reason for it in the way we actually pronounce the letters."